In a circuit that drives a load by delivering constant current, a current value can be calculated by reading a voltage drop of a resistor through which the energizing path passes. Based on the fact that constant current flows when the circuit is functioning normally, the circuit can be determined to be in a normal state. Likewise, a possibility of a disconnection failure can be detected when the current value is short of a constant current level.
In the circuit in which constant current is delivered, the resistance value of the load is adjusted by means of a current-limiting resistor connected in series to the load, so that constant current is delivered by a power supply voltage. Accordingly, when the power supply voltage is low, the resistance value of the current-limiting resistor is reduced, which in turn reduces the amount of voltage drop at both ends of the resistor when the load is energized. As a result, the change in the level of detected voltage between when the circuit is in an on state and when the circuit is in an off state will be small. A high precision amplifier needs to be added in order to detect such a change accurately, which will lead to higher cost.
Even in a configuration provided with a high precision amplifier, there is a possibility that the detected voltage is obscured and causes erroneous determination, when the power supply voltage that supplies power to the load fluctuates and the fluctuations in the power supply voltage are large.